1. Field of the Invention
The present invention relates to a method for examining a sample surface using an atomic force scanning microscope comprising a cantilever with a longitudinal extension, along which the measuring tip is arranged precisely relative to a sample surface by means of a means for driving, the spatial position of the measuring tip being determined by a sensor unit. The microscope is further provided with at least one ultrasound generator with which vibration excitation is initiated at a given excitation frequency between the sample surface and the cantilever. The measuring tip of the cantilever is brought into contact with the sample surface in such a manner that the oscillations imparted to the measuring tip are oriented lateral to the sample surface and perpendicular to the length of the cantilever. The torsional vibrations induced in the cantilever are detected and analyzed by means of an evaluation unit.
2. Description of the Prior Art
The development of an atomic force scanning microscope has permitted major achievements in the field of examination of surface properties, in particular in the characterization of surface properties. For the first time, it is possible to obtain information concerning surfaces and areas close to the surface of very different samples in nanometer resolution even in the magnitude of single atoms. Friction force microscopy, a further development of the atomic force scanning microscope, permitted for the first time studying one of the oldest problems in science, the examination of friction, on this scale.
DE 43 24 983 C2 describes an acoustical microscope operating on the technological basis of an atomic force scanning microscope that is able to measure the topography as well as the elastic properties of the surface of a sample. The microscope comprises a cantilever designed as a leaf spring, usually with a length of between 100 μm and 500 μm, attached to the one end of which is a pyramid-shaped measuring tip having a tip radius of curvature of about 50 nanometers.
In order to measure and examine the sample surface holistically, the cantilever and the measuring tip attached thereto are scanned over the sample surface with the aid of a suitable means for moving in such a manner that the measuring tip makes contact with the sample surface with a given vertical load at every single scanning point. The optical sensor unit permits determining the degree of deformation of the cantilever and thus the topography-based excursion of the measuring tip. Usually, the optical sensor unit is provided with a laser diode from which a laser beam directed at the cantilever is emitted, reflected thereat, and detected by a position-sensitive photodiode. During scanning, the cantilever and the measuring tip are guided perpendicular to the sample surface via a regulation loop in such an active manner that the excursion of the cantilever, and the vertical load with which the cantilever lies on the sample surface via the measuring tip, remains constant. The regulation tension required for the excursion is usually converted into a distance value and is correspondingly depicted as an encoded color value in a representation showing the surface topography.
In order to also be able to determine the elastic properties of the surface sample, an ultrasound generator is provided which induces oscillations in the surface sample while the measuring tip lies at a scanning point of the sample surface. Vibration excitation by coupling in ultrasonic waves leads to normal vibrations of the sample surface which induce high-frequency oscillating bending vibrations in the cantilever along its longitudinal extension.
Detection by the ultrasonically induced, high-frequency vibration behavior of the cantilever permits obtaining information about the elastic properties of the sample surface. The problem with this measuring situation that needs to be resolved lies in the decoupling due to the measurement of the superimposed excursions of the cantilever, which result, on the one hand, from the topography measurement due to which the vertical load with which the measuring tip lies on the sample surface remains, as constant as possible and, on the other hand, which cause the ultrasonically induced normal vibrations of the sample surface transmitted to the cantilever via the measuring tip.
In order to obtain a reliable measuring signal with a high as possible signal/noise ratio for measuring the elasticity, the ultrasonically induced vibration excitation of the sample surface occurs at frequencies which are at least one magnitude greater than the resonant frequency of the cantilever having the measuring tip attached thereto. Using two photodiodes with different temporal responding behavior, on which the light beam reflected at the cantilever impinges, permits selective detection and evaluation of the vibration behavior of the cantilever. Thus, the photodiode with a slow response behavior is able to solely detect the excursions resulting from the contour-based readjustment of the cantilever for determining the topography. On the other hand, the second photodiode, which has a bandwidth window in the MHz range, is provided for determining the high-frequency vibration parts of the cantilever. Especially suited therefor are, for example, single-cell light-sensitive detectors with a smooth-edged means for shading, for example in the form of a razor blade or a so-called heterodyne running-time interferometer, in the one interferometer arm of which a frequency shift means is provided. Such a rapid type responding detection unit can also be designed based on a capacity measurement, in which the measuring capacity is formed from the cantilever and a needle-shaped counter-electrode disposed opposite thereto. Further details can be found in the aforementioned printed publication DE 43 24 983 C2.
Contrary to the aforedescribed resonance measurement with vertical modulation, that is the to-be-examined sample surface is excited to normal vibrations U.S. Pat. No. 5,804,708 describes an atomic force microscope with a similar setup, but vibration excitation of the to-be-examined sample occurs with the aid of a signal generator in such a manner that the sample surface imparts vibrations oriented lateral to the sample surface and, in particular, directed transverse in relation to the longitudinal extension of the cantilever.
The vibration excitation directed transverse to the longitudinal extension of the cantilever induces torsional vibrations in the cantilever in contact with the sample surface via the measuring tip, with the measuring tip, which is at least sometimes in contact with the sample surface, executing oscillations which are directed in longitudinal direction to the sample surface and transverse to the longitudinal extension of the cantilever, respectively are polarized. The measuring tip briefly adheres to the sample surface at the point of reversal of the oscillations. The sample surface is deformed by the shear forces acting laterally to the sample surface until, due to friction, the measuring tip slips out of the described state back over the sample surface.
The shear deformations formed at the returning points in dependence on the vertical load with which the measuring tip lies on the sample surface influence the vibration behavior of the measuring tip and consequently that of the cantilever in a manner which characterizes the elastic properties of the sample surface. Thus, it is possible to obtain information about the elastic properties of the sample surface from the vibration behavior, for example from the vibration amplitude and/or the phase of the oscillations occurring in the form of torsional vibrations along the cantilever.
The oscillations initiated by the signal generator in the sample have frequencies of approximately 1 kHz. However, with this measuring method, local resolution has proven unsatisfactory. Only measurements with a local resolution of approximately 100 nm can be achieved. Moreover, the measuring quality achievable with this method permits obtaining only qualitative information about the frictional properties of the sample surface.